1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device and, more specifically, to an integrated circuit (IC) including an analog circuit and a digital circuit together.
2. Description of the Related Art
In a prior art integrated circuit (IC) including both an analog circuit and a digital circuit, a digital signal having a large amplitude and an analog signal having a small amplitude are processed on the single IC. It is thus necessary to prevent the digital signal from leaking into the analog signal. For example, the leakage of a signal through a substrate can be prevented by acquiring a substrate potential from each of analog and digital circuits or by forming a number of substrate contacts.
In the above integrated circuit, the wiring length is increased in accordance with a reduction in wiring interval and an improvement in degree of integration of IC; accordingly, a crosstalk is caused between wirings owing to a signal leakage therebetween. To prevent the crosstalk, as shown in FIG. 1, a ground (GND) line 5 is formed above wiring 3, which is formed on an insulation film 2 on the major surface of a substrate 1 and serves as a signal line, with an insulation film 4 interposed between the wiring 3 and GND line 5. The wiring (signal line) 3 is therefore covered with the GND line 5.
However, in the prior art integrated circuit described above, the parasitic capacitance between the wiring 3 and GND line 5 is increased when the wiring 3 is long or wide. Since the resistance of a high-frequency analog signal is usually represented by 1/j.omega.C (.omega.: radian frequency, C: capacitance), the components of the high-frequency analog signal attenuate as the capacitance increases. Furthermore, the potential of the signal is likely to change because of a leak of small current flowing between wirings.
As described above, the prior art integrated circuit has a drawback in which when the wiring layer is covered with the GN line to prevent a crosstalk from occurring between the wires, the components of the high-frequency analog signal may attenuate or the potential of the signal may change.